The present invention relates Fe-Cr-Mn-N stainless steels with reduced levels of Ni which develop high levels of strength and have improved castability.
Due to the cost of nickel, a considerable interest has developed to lower nickel in austenitic stainless steels. The elements normally used to replace nickel include manganese, nitrogen, carbon and copper. The balance between these nickel replacement elements is important to develop the desired mechanical properties in addition to maintaining an austenitic structure.
The composition balance for Fe-Cr-Mn-N steels depends on the need for weldability, corrosion resistance, toughness, wear, strength and other properties. Regardless of the desired properties, the composition must be essentially austenite in the annealed, cast or unworked condition.
U.S. Pat. No. 3,989,474 (G. N. Goller et al.) discloses an austenitic stainless steel article consisting essentially of 0.06 to 0.12% carbon, 11 to 14% manganese, 15.5 to 20% chromium, 1.1 to 2.5% nickel, 0.2 to 0.38% nitrogen, 1% maximum silicon and balance iron. The steels are characterized by high strength levels after working and they remain substantially fully austenitic.
British Patent No. 2,075,550 (Douthett et al.) is an inexpensive austenitic stainless steel balanced for high work hardening and good-abrasion resistance. The composition range disclosed consists essentially of 0.015 to 0.10% carbon, 6 to 10% manganese, 13 to 20% chromium, 1 to 3% nickel, 0.15 to 0.22% nitrogen, 2% maximum silicon and balance iron. The desired properties are developed by controlling the austenite stability to obtain deformation martensite during cold reduction.
Fulmer Research Laboratories Ltd. has done work recently to adjust the balance in Fe-Cr-Mn-N steels for applications in hard-rock mines. The stability of the austenite was reduced and the martensite start temperatures were investigated by Fulmer and reported in Metallurgical Transactions, Volume 18A, May 1987, pages 767-775. The steels investigated were 8-12% Cr, 0-10% Mn, 0-0.6% N and about 0.02% max C. The nitrogen was introduced by high temperature nitriding. A companion Fulmer paper was published in the same issue of Metallurgical Transactions which discussed the properties of related steels up to 0.049% carbon (pages 847-855).
The prior work by others has not dealt with the problems which develop during casting of an Fe-Cr-Mn-N alloy. The composition has not been balanced for improving castability. The present invention has discovered a composition which has improved the strength and ductility during high temperature continuous casting for improved yields. The steels must also be balanced to avoid thermal transformation to martensite and have a high work hardening rate which provides high tensile strengths and improved ductility compared to prior alloys after cold working. The stainless steels of the past could not provide these properties with a low-cost austenitic stainless steel composition.